Quick Answer
The answer is MP-BGP, as it is the only protocol capable of distributing VPN labels in an MPLS VPN architecture. MP-BGP carries VPN-IPv4 routes within its Network Layer Reachability Information (NLRI), embedding both the route distinguisher (RD) and the VPN label—often called the service label—so that Provider Edge (PE) routers can exchange per-VRF label bindings. This enables label-switched VPN traffic across the MPLS core, even when PEs are connected through intermediate P routers. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how control-plane signaling separates VPN-specific labels from transport LSPs; a common trap is confusing LDP or RSVP-TE for VPN label distribution, but those only handle the core IGP labels. Remember the memory tip: "VPN labels ride MP-BGP, not LDP"—the service label is always carried in the BGP update, while the transport label comes from the IGP.
This 350-501 practice question tests your understanding of mpls and segment routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer is designing an MPLS network and needs to ensure that VPN traffic between two PE routers is label-switched. The PE routers are connected via a P router and have a full mesh of iBGP sessions. Which label distribution method is required for the VPN labels?
Segment Routing
Why wrong: SR can be used for transport but VPN labels are still advertised via BGP.
MP-BGP
MP-BGP carries VPNv4 routes with MPLS labels.
RSVP-TE
Why wrong: RSVP-TE is for TE tunnels, not for VPN label distribution.
LDP
Why wrong: LDP assigns labels to IGP routes, not VPN prefixes.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
MP-BGP
In MPLS VPN architectures, VPN labels (also known as service labels) are distributed using Multiprotocol BGP (MP-BGP). MP-BGP carries VPN-IPv4 routes that include both the route distinguisher (RD) and the VPN label in the Network Layer Reachability Information (NLRI). This allows PE routers to exchange per-VRF label bindings, enabling label-switched VPN traffic across the MPLS core. Without MP-BGP, the PE routers cannot signal the VPN-specific labels required for end-to-end label switching.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Segment Routing
MP-BGP
Why this is correct
MP-BGP carries VPNv4 routes with MPLS labels.
Related concept
Read the scenario before looking for a memorised answer.
RSVP-TE
Why it's wrong here
RSVP-TE is for TE tunnels, not for VPN label distribution.
LDP
Why it's wrong here
LDP assigns labels to IGP routes, not VPN prefixes.
Common exam traps
Cisco often tests the distinction between transport labels (distributed by LDP or RSVP-TE) and service labels (distributed by MP-BGP), leading candidates to incorrectly choose LDP or RSVP-TE for VPN label distribution.
Detailed technical explanation
MP-BGP for VPNs is defined in RFC 4364, where the VPN label is encoded as a 3-byte field in the MP_REACH_NLRI attribute. The label is allocated per-VRF and per-prefix, allowing the egress PE to demultiplex traffic into the correct VRF. In real-world deployments, a common misconfiguration is forgetting to enable the 'label' capability under the address-family vpnv4, which prevents VPN label exchange even if MP-BGP sessions are up.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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MPLS and Segment Routing — study guide chapter
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
The correct answer is: MP-BGP — In MPLS VPN architectures, VPN labels (also known as service labels) are distributed using Multiprotocol BGP (MP-BGP). MP-BGP carries VPN-IPv4 routes that include both the route distinguisher (RD) and the VPN label in the Network Layer Reachability Information (NLRI). This allows PE routers to exchange per-VRF label bindings, enabling label-switched VPN traffic across the MPLS core. Without MP-BGP, the PE routers cannot signal the VPN-specific labels required for end-to-end label switching.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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